The present invention relates to a fiber optic cable having flame retardance capabilities.
Conventional fiber optic cables comprise optical fibers which are used to transmit voice, video, and data information. Indoor and indoor/outdoor fiber optic cables have been developed for installation in plenums, risers, and ducts of buildings. Fiber optic cables suitable for indoor use are required to meet flame retardance standards for the prevention, inhibition, and/or extinguishment of flame. A cable's ability to prevent, inhibit, or extinguish the spread of flames is determined by means of horizontal and vertical flame tests. In addition to flame tests, indoor/outdoor cables must meet water blocking requirements for the prevention of the flow of water therein.
Materials used for the prevention, inhibition, and/or extinguishment of flame may fall into two general categories. The first category includes inherently inflammable, flame-resistant materials which are thermally stable, and may have high decomposition temperatures, for example, certain metals or high temperature plastics. The materials included in the first category are useful as thermal/heat/flame barriers. Thermal/heat/flame barriers may have disadvantages, namely: they are generally expensive; and, because of limited burn-performance characteristics, they can be used in but a narrow range of applications. The second general category of materials used for the prevention, inhibition, and/or extinguishment of flame includes inherently flammable materials which have been chemically altered to include flame inhibiting agents, which agents actively interfere with the chemical reactions associated with combustion. Examples of inherently flammable materials are polyethylene, polypropylene, polystyrene, polyesters, polyurethanes, and epoxy resins. By comparison, thermal/heat/flame barriers typically do not include flame inhibiting agents, but rather are relied upon in flame protection designs for their resistance to decomposition at high temperatures, or their inherent heat dissipation or flame barrier properties.
An example of a fiber optic cable with some inherent thermal/heat resistance ability is disclosed in US-A 5261021, which cable includes a tape wrapped around conductors. The tape comprises a corrugated metal strip, a layer of adhesive, and a superabsorbent powder layer applied to the adhesive layer, which layers collectively comprise a thermal/heat/flame barrier. The tape is then wrapped around the conductors so that longitudinally extending edges of the tape are positioned in overlapping engagement. Although this known fiber optic cable has some thermal/heat resistance and water blocking features, this design has several disadvantages. For example, the tape requires three layers of material, the collective thicknesses and stiffnesses of which layers result in a large, heavy, and stiff fiber optic cable. The size, weight, and stiffness of this known fiber optic cable make the cable difficult to route through cable passageways during installation. Additionally, the manufacture of the tape requires the purchase and preparation of the metal strip, adhesive, and water blocking powder, and the combination thereof into a tape structure. The expense of manufacturing the tape necessarily contributes to the cost of production of the fiber optic cable. Moreover, although the metal layer may dissipate heat, it is typically grounded during installation.
An example of a fiber optic cable having a non-metallic thermal/heat/flame barrier suitable for indoor use is disclosed in U.S. Pat. No. 5,566,266. This known cable is an optical service cable with a core tube having a stack of optical fiber ribbons. The core tube is filled with a hydrophobic water-blocking compound having a flame retardant material mixed therein. The known cable further includes two layers of water swellable tape, and a layer comprising a flame resistant tape. The flame resistant tape is made of a polyimide, e.g. a KAPTON or a TEFLON material. The two water-swellable tape layers and the flame resistant tape layer have associated material costs which contribute to the cost of production of the fiber optic cable. Moreover, the respective thicknesses and stiffnesses of the layers contribute to the overall size and stiffness of the cable which may make the cable difficult to route through passageways.
A thermal/heat/flame barrier for blocking heat flow into a cable core during a lightning strike is disclosed in U.S. Pat. No. 5,131,064. This known fiber optic cable is designed for use in outside plant environments and includes a core comprising optical fiber ribbons and a mechanically strengthened, thermal barrier layer disposed about a plastic tubular member. A metallic shield and a plastic jacket surround the thermal barrier layer. The thermal barrier layer comprises a temperature resistant tape which is made of a woven glass or aramid fibrous material. The thermal barrier layer is a laminate comprising a high temperature resistant tape and at least one other tape with a superabsorbent powder thereon. Although the known fiber optic cable has thermal and water blocking protection, this design has several disadvantages. For example, the laminate requires a relatively thick woven glass or aramid fiber layer, and at least one water blocking layer, the respective thicknesses and stiffnesses of which layers combine to result in a large, heavy, and stiff fiber optic cable. The size, weight, and stiffness of this known fiber optic cable make the cable difficult to route through cable passageways during installation. Additionally, manufacture of the cable requires the purchase, preparation, and lamination of the woven glass or aramid fiber and water blocking layers, which contributes to the cost of production of the cable.
Other thermal/heat/flame barriers used in fiber optic cables include: a KEVLAR tape as disclosed in U.S. Pat. No. 4,143,942; a TEFLON tape as disclosed in U.S. Pat. No. 5,185,840; and a layer of a multi-layer buffer tube wall, which layer includes hollow glass fillers as disclosed in U.S. Pat. No. 5,495,546. The respective thermal/heat/flame barriers of the foregoing fiber optic cables do not include flame inhibiting agents. Moreover, the respective sizes, weights, and stiffnesses of the barriers may make the cables difficult to route through cable passageways during installation. Additionally, the KEVLAR and TEFLON materials and specialized buffer tube layer with glass fillers may be expensive.
The second general category of materials used for the prevention, inhibition, and/or extinguishment of flame, as noted hereinabove, includes inherently flammable materials which have been chemically altered to include flame inhibiting agents. With respect to conventional fiber optic cables and optical fibers, the materials in the jackets of the cables and optical fibers are modified to include one or more flame inhibiting agents. Fiber optic cable jackets are different from substrates because they represent a mass of material which is a solid cross section, not including interstices.
A known fiber optic cable jacket which includes an active flame inhibiting agent is disclosed in U.S. Pat. No. 5,358,011, wherein the jacket includes a polyolefin compound Megolon S300. This compound is a blend of co-polymers with an inorganic compound included as a flame inhibiting agent, namely, aluminum trihydrate. U.S. Pat. No. 5,133,034 discloses a jacket formed of a polyolefin based material including a metal hydroxide filler as a flame inhibiting agent.
U.S. Pat. No. 5,136,683 discloses a plastic optical fiber with a common matrix jacket formed of a flame retardant material having an Oxygen Index (OI) of at least 32. The OI allows a simple determination of a material's flammability, it is a measure of a material's relative flammability as compared to other materials that are capable of burning in oxygen, i.e. the percentage of oxygen in a gas needed to support combustion. A jacket material with a high OI is desirable because it indicates low flammability. Jacket materials disclosed in U.S. Pat. NO. 5,136,683 which may have an acceptable OI are chlorinated polyethylene, polyethylene, PVC, an EVA-type polymer, a water-crosslinked polyolefin, polyvinylidene chloride, polyvinylidene fluoride, polyfluoroethylene, and other halogen containing polymers. Flame inhibiting agents for chemically altering jacket materials may include, for example, tetrabromoethane, chlorinated paraffin, chlorinated polyethylene, tetrabromobisphenol A, and phosphate compounds. The preferred inorganic compounds are indicated as being antimony trioxide and aluminum hydroxide. Another plastic optical fiber is disclosed in U.S. Pat. No. 5,206,926, wherein the plastic fiber is described as including a jacket material comprising a single polymer or a mixture of polymers with a flame inhibiting additive having an OI of at least 25.
Polyvinyl chloride (PVC) as a pure polymer is inherently flame inhibiting because of its high chlorine content but plasticizers are typically flammable. Known fiber optic cables which include PVC as a jacket material are U.S. Pat. No. 5,345,525 and U.S. Pat. No. 5,253,318. PVC may not, however, be a highly desirable jacketing material in all circumstances because of the potential for the evolution of unacceptable levels of toxic gases during burning. Moreover, the foregoing cable and optical fiber jackets do not include water blocking capabilities for preventing the longitudinal flow of water.
A known electrical cable having a flame inhibiting agent mixed with a water blocking substance is described in U.S. Pat. No. 3,944,717. The electrical cable includes a core with insulated electrical conductors. During manufacture of the electrical cable, the entire cross section of the core is advanced through a bath of a filling composition including a chlorinated paraffin for flame retardancy and water blocking, a polyvinyl chloride resin base for viscosity, and a chlorinated polyethylene for adherence to insulated conductors. Although this known electrical cable has flame inhibiting and water blocking capabilities, the design thereof has several disadvantages. For example, this cable is not craft-friendly because the filling composition makes the cable heavy and therefore difficult to route through passageways during installation, and the filling composition is difficult to remove from the conductors. Additionally, manufacture of the cable requires the purchase, preparation, and processing of the chlorinated paraffin, polyvinyl chloride resin base, and chlorinated polyethylene, which altogether contribute to the production costs of the cable. Also, the filling compound may not reach all of the interstices between the conductors thereby creating the potential for leak-paths. Furthermore, compared to the optical fibers of a fiber optic cable, the known electrical cable is disadvantageous because the electrical conductors thereof have smaller bandwidths than optical fibers, and the electrical conductors are subject to higher power loss. Moreover, the electrical conductors are subject to electromagnetic interference, impedance, and electrical cross talk. Further, electrical cables are generally heavier and larger than comparable fiber optic cables, making electrical cables comparatively more difficult to install than optical fiber cables. Finally, because the known electrical cable emits electromagnetic energy, it is easier to wire-tap and is therefore less secure than a fiber optic cable.